Thin film transistor, method for manufacturing the same, and semiconductor device

ABSTRACT

In a thin film transistor, an increase in off current or negative shift of the threshold voltage is prevented. In the thin film transistor, a buffer layer is provided between an oxide semiconductor layer and each of a source electrode layer and a drain electrode layer. The buffer layer includes a metal oxide layer which is an insulator or a semiconductor over a middle portion of the oxide semiconductor layer. The metal oxide layer functions as a protective layer for suppressing incorporation of impurities into the oxide semiconductor layer. Therefore, in the thin film transistor, an increase in off current or negative shift of the threshold voltage can be prevented.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to a thin film transistor formed using anoxide semiconductor layer, and a method for manufacturing the thin filmtransistor. In addition, the present invention relates to asemiconductor device manufactured using the thin film transistor.

Note that a semiconductor device in this specification indicates all thedevices that can operate by using semiconductor characteristics, andelectro-optical devices, semiconductor circuits, and electronicappliances are all included in the category of the semiconductordevices.

2. Description of the Related Art

A wide variety of metal oxides exist and are used for variousapplications. Indium oxide is a well-known material and is used as atransparent electrode material needed for a liquid crystal display andthe like.

Some metal oxides exhibit semiconductor characteristics. As metal oxidesexhibiting semiconductor characteristics, for example, tungsten oxide,tin oxide, indium oxide, zinc oxide, and the like can be given.References disclose a thin film transistor in which such a metal oxideexhibiting semiconductor characteristics is used for a channel formationregion (Patent Documents 1 to 4, and Non-Patent Document 1).

As metal oxides, multi-component oxides as well as single-componentoxides are known. For example, InGaO₃(ZnO)_(m) (m is a natural number)belonging to homologous series has been known as a multi-component oxidesemiconductor including In, Ga, and Zn (Non-Patent Documents 2 to 4).

In addition, it has been confirmed that an oxide semiconductor includingsuch an In—Ga—Zn-based oxide can be used as a channel layer of a thinfilm transistor (Patent Document 5, and Non-Patent Documents 5 and 6).

REFERENCE Patent Document

-   [Patent Document 1] Japanese Published Patent Application No.    S60-198861-   [Patent Document 2] Japanese Published Patent Application No.    H8-264794-   [Patent Document 3] Japanese Translation of PCT International    Application No. H11-505377-   [Patent Document 4] Japanese Published Patent Application No.    2000-150900-   [Patent Document 5] Japanese Published Patent Application No.    2004-103957

Non-Patent Document

-   [Non-Patent Document 1] M. W. Prins, K. O. Grosse-Holz, G.    Muller, J. F. M. Cillessen, J. B. Giesbers, R. P. Weening, and R. M.    Wolf, “A ferroelectric transparent thin-film transistor”, Appl.    Phys. Lett., 17 June, 1996, Vol. 68, pp. 3650-3652-   [Non-Patent Document 2] M. Nakamura, N. Kimizuka, and T. Mohri, “The    Phase Relations in the In₂O₃—Ga₂ZnO₄—ZnO System at 1350° C.”, J.    Solid State Chem., 1991, Vol. 93, pp. 298-315-   [Non-Patent Document 3] N. Kimizuka, M. Isobe, and M. Nakamura,    “Syntheses and Single-Crystal Data of Homologous Compounds,    In₂O₃(ZnO)m (m=3, 4, and 5), InGaO₃(ZnO)₃, and Ga₂O₃(ZnO)_(m) (m=7,    8, 9, and 16) in the In₂O₃—ZnGa₂O₄—ZnO System”, J. Solid State    Chem., 1995, Vol. 116, pp. 170-178-   [Non-Patent Document 4] M. Nakamura, N. Kimizuka, T. Mohri, and M.    Isobe, “Syntheses and crystal structures of new homologous    compounds, indium iron zinc oxides (InFeO₃(ZnO)_(m)) (m: natural    number) and related compounds”, KOTAI BUTSURI (SOLID STATE PHYSICS),    1993, Vol. 28, No. 5, pp. 317-327-   [Non-Patent Document 5] K. Nomura, H. Ohta, K. Ueda, T. Kamiya, M.    Hirano, and H. Hosono, “Thin-film transistor fabricated in    single-crystalline transparent oxide semiconductor”, SCIENCE, 2003,    Vol. 300, pp. 1269-1272-   [Non-Patent Document 6] K. Nomura, H. Ohta, A. Takagi, T. Kamiya, M.    Hirano, and H. Hosono, “Room-temperature fabrication of transparent    flexible thin-film transistors using amorphous oxide    semiconductors”, NATURE, 2004, Vol. 432, pp. 488-492

SUMMARY OF THE INVENTION

An object of an embodiment of the present invention is to prevent, in athin film transistor, an increase in off current or negative shift ofthe threshold voltage.

An object of an embodiment of the present invention is to realize ohmiccontact between an oxide semiconductor layer and each of a sourceelectrode layer and a drain electrode layer of a thin film transistor.

An object of an embodiment of the present invention is to efficientlymanufacture a high-performance thin film transistor in which an increasein off current or negative shift of the threshold voltage is prevented.

An object of an embodiment of the present invention is to efficientlymanufacture a high-performance thin film transistor in which an increasein off current or negative shift of the threshold voltage is preventedand which has ohmic contact between an oxide semiconductor layer andeach of a source electrode layer and a drain electrode layer.

An object of an embodiment of the present invention is to provide asemiconductor device with high performance or high reliability.

An embodiment of the present invention is an inverted staggered thinfilm transistor in which a buffer layer is provided over an oxidesemiconductor layer and a source electrode layer and a drain electrodelayer are provided over the buffer layer. Note that the buffer layerincludes a pair of conductive layers provided over opposite end portionsof the oxide semiconductor layer, and a metal oxide layer which isprovided over a middle portion of the oxide semiconductor layer, has thesame metal element as the pair of conductive layers, has higher oxygenconcentration than the pair of conductive layers, and is an insulator ora semiconductor provided between the pair of conductive layers.

Another embodiment of the present invention is a thin film transistorwhich has the above structure and in which the buffer layer includes apair of oxide semiconductor layers whose oxygen concentration is loweredand which are provided over opposite end portions of the oxidesemiconductor layer, and a pair of conductive layers containing oxygenat high concentration and provided over the pair of oxide semiconductorlayers whose oxygen concentration is lowered.

Note that in this specification, the term “insulator” means a substancewhose electrical resistivity is greater than or equal to 10⁶ (Ω·m), theterm “semiconductor” means a substance whose electrical resistivity isgreater than or equal to 10⁻³ (Ω·m) and less than 10⁶ (Ω·m), and theterm “conductor” means a substance whose electrical resistivity is lessthan 10⁻³ (Ω·m).

Another embodiment of the present invention is a method formanufacturing a thin film transistor in which a metal oxide layer isformed by performing oxidation treatment on a conductive layer formed inthe same step as an oxide semiconductor layer. Note that in theoxidation treatment, a resist used for forming a source electrode layerand a drain electrode layer is used as a mask. Therefore, opposite endportions of the conductive layer remain without being oxidized by theoxidation treatment. As a result, through the oxidation treatment, apair of conductive layers and a metal oxide layer provided between thepair of conductive layers are formed.

Another embodiment of the present invention is a method formanufacturing a thin film transistor in which a metal oxide layer isformed by performing oxidation treatment on a conductive layer formed inthe same step as an oxide semiconductor layer, and then, a pair ofconductive layers containing oxygen at high concentration and a pair ofoxide semiconductor layers whose oxygen concentration is lowered areformed by diffusing oxygen through thermal treatment.

Another embodiment of the present invention is a method formanufacturing a thin film transistor in which a metal oxide layer, apair of conductive layers containing oxygen at high concentration, and apair of oxide semiconductor layers whose oxygen concentration is loweredare formed through thermal oxidation treatment.

Another embodiment of the present invention is a method formanufacturing a thin film transistor in which a metal oxide layer, apair of conductive layers containing oxygen at high concentration, and apair of oxide semiconductor layers whose oxygen concentration is loweredare formed through oxidation treatment and thermal oxidation treatment.

Another embodiment of the present invention is a semiconductor deviceincluding the thin film transistor, and an interlayer insulating layerprovided over the thin film transistor.

An embodiment of the present invention is an inverted staggered thinfilm transistor including a metal oxide layer which is an insulator or asemiconductor over a middle portion of an oxide semiconductor layer. Themetal oxide layer functions as a protective layer for suppressingincorporation of impurities (such as hydrogen and moisture) into theoxide semiconductor layer. Therefore, in the thin film transistor, anincrease in off current or negative shift of the threshold voltage canbe prevented.

Another embodiment of the present invention is an inverted staggeredthin film transistor including a pair of conductive layers containingoxygen at high concentration and a pair of oxide semiconductor layerswhose oxygen concentration is lowered, between opposite end portions ofan oxide semiconductor layer and a pair of conductive layers providedover the opposite end portions of the oxide semiconductor layer. Thepair of oxide semiconductor layers whose oxygen concentration is loweredhas lower resistance than the oxide semiconductor layer. Therefore,ohmic contact between the oxide semiconductor layer and each of a sourceelectrode layer and a drain electrode layer can be obtained.

In another embodiment of the present invention, the metal oxide layer isformed on the basis of a conductive layer which is formed in the samestep as the oxide semiconductor layer. Therefore, a high-performancethin film transistor can be efficiently formed.

In another embodiment of the present invention, the metal oxide layer isformed on the basis of a conductive layer which is formed in the samestep as the oxide semiconductor layer, and the pair of oxidesemiconductor layers whose oxygen concentration is lowered is formed bydiffusing oxygen into the conductive layer. Therefore, ahigh-performance thin film transistor can be efficiently formed.

In another embodiment of the present invention, a thin film transistorincluding a protective layer for suppressing incorporation of impurities(such as hydrogen and moisture) into an oxide semiconductor layer isapplied to a thin film transistor included in a semiconductor device.Accordingly, a material and a manufacturing method of an interlayerinsulating layer provided over the thin film transistor can be selecteddepending on the purpose. That is, a semiconductor device with highperformance or high reliability can be provided.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are cross-sectional views each illustrating a thin filmtransistor described in Embodiment 1.

FIGS. 2A to 2D are cross-sectional views illustrating a manufacturingprocess of a thin film transistor described in Embodiment 2.

FIGS. 3A to 3D are cross-sectional views illustrating a manufacturingprocess of a thin film transistor described in Embodiment 2.

FIG. 4 is a top view illustrating a pixel of a liquid crystal displaydevice described in Embodiment 3.

FIG. 5 is a cross-sectional view illustrating a pixel of a liquidcrystal display device described in Embodiment 3.

FIG. 6 is an equivalent circuit diagram illustrating a pixel of a liquidcrystal display device described in Embodiment 3.

FIG. 7 is a top view illustrating a pixel of a light-emitting displaydevice described in Embodiment 4.

FIG. 8 is a cross-sectional view illustrating a pixel of alight-emitting display device described in Embodiment 4.

FIG. 9 is an equivalent circuit diagram illustrating a pixel of alight-emitting display device described in Embodiment 4.

FIG. 10 is a cross-sectional view illustrating an electronic paperdescribed in Embodiment 5.

FIGS. 11A to 11C are graphs each showing density of a state obtained bycalculation described in Example 1.

FIGS. 12A to 12C are graphs each showing density of a state obtained bycalculation described in Example 1.

FIGS. 13A and 13B are graphs each showing density of a state obtained bycalculation described in Example 1.

FIGS. 14A and 14B are diagrams showing atomic arrangement before andafter thermal treatment at a bonding interface between a titanium layerand an In—Ga—Zn—O-based oxide semiconductor layer, which are obtained bycalculation described in Example 1.

FIG. 15 is a graph showing titanium concentration and oxygenconcentration before and after thermal treatment at a bonding interfacebetween a titanium layer and an In—Ga—Zn—O-based oxide semiconductorlayer, which are obtained by calculation described in Example 1.

FIGS. 16A and 16B are diagrams showing atomic arrangement before andafter thermal treatment at a bonding interface between a titanium oxidelayer and an In—Ga—Zn—O-based oxide semiconductor layer, which areobtained by calculation described in Example 1.

FIG. 17 is a graph showing titanium concentration and oxygenconcentration before and after thermal treatment at a bonding interfacebetween a titanium oxide layer and an In—Ga—Zn—O-based oxidesemiconductor layer, which are obtained by calculation described inExample 1.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments and an example of the present invention will be hereinafterdescribed in detail with reference to the accompanying drawings.However, the present invention is not limited to the description below,and those skilled in the art will appreciate that a variety ofmodifications can be made to the modes and details without departingfrom the spirit and scope of the present invention. Therefore, thepresent invention is not construed as being limited to description ofembodiments and an example below.

Note that the size, the thickness of a layer, or a region of eachstructure illustrated in drawings or the like in embodiments isexaggerated for simplicity in some cases. Therefore, the scale is notnecessarily limited to that illustrated in the drawings or the like.Further, in this specification, ordinal numbers such as “first”,“second”, and “third” are used in order to avoid confusion amongcomponents, and the terms do not limit the components numerically.

Embodiment 1

In this embodiment, a structure of a thin film transistor which is oneembodiment of the present invention will be described with reference toFIGS. 1A and 1B. Next, characteristics of the thin film transistor willbe described.

FIG. 1A illustrates a cross-sectional view of a thin film transistor 150formed over a substrate 100. The thin film transistor 150 includes agate electrode layer 101 provided over the substrate 100, a gateinsulating layer 102 provided over the gate electrode layer 101, anoxide semiconductor layer 103 provided over the gate insulating layer102, a buffer layer 106 provided over the oxide semiconductor layer 103and including a pair of conductive layers 104 a and 104 b which areconductors and a metal oxide layer 105 which is an insulator or asemiconductor, a source electrode layer 107 a provided over theconductive layer 104 a (one of the pair of conductive layers 104 a and104 b), and a drain electrode layer 107 b provided over the conductivelayer 104 b (the other of the pair of conductive layers 104 a and 104b). Note that the pair of conductive layers 104 a and 104 b is providedover opposite end portions of the oxide semiconductor layer 103, and themetal oxide layer 105 is provided over a middle portion of the oxidesemiconductor layer 103.

In other words, the thin film transistor 150 in FIG. 1A is an invertedstaggered thin film transistor including the buffer layer 106 whichincludes the pair of conductive layers 104 a and 104 b and the metaloxide layer 105 and is provided between the oxide semiconductor layer103 and each of the source electrode layer 107 a and the drain electrodelayer 107 b.

FIG. 1B illustrates a cross-sectional view of a thin film transistor 151formed over a substrate. The thin film transistor 151 has a structure ofthe thin film transistor 150 in FIG. 1A. In addition, the thin filmtransistor 151 includes a pair of oxide semiconductor layers 108 a and108 b whose oxygen concentration is lowered and which are provided overthe opposite end portions of the oxide semiconductor layer 103, and apair of conductive layers 109 a and 109 b containing oxygen at highconcentration and provided over the pair of oxide semiconductor layers108 a and 108 b whose oxygen concentration is lowered.

In other words, the thin film transistor 151 in FIG. 1B is an invertedstaggered thin film transistor including a buffer layer 110 which isprovided between the oxide semiconductor layer 103 and each of thesource electrode layer 107 a and the drain electrode layer 107 b. Thebuffer layer 110 includes the pair of conductive layers 104 a and 104 b,the metal oxide layer 105, the pair of oxide semiconductor layers 108 aand 108 b whose oxygen concentration is lowered, and the pair ofconductive layers 109 a and 109 b containing oxygen at highconcentration.

As the substrate 100, a glass substrate such as barium borosilicateglass or aluminoborosilicate glass or the like can be used.

As the gate electrode layer 101, an element selected from aluminum (Al),copper (Cu), titanium (Ti), tantalum (Ta), tungsten (W), molybdenum(Mo), chromium (Cr), neodymium (Nd), and scandium (Sc), an alloycontaining any of these elements, or a nitride containing any of theseelements can be used. A stacked structure of these materials can also beused.

As the gate insulating layer 102, an insulator such as silicon oxide,silicon nitride, silicon oxynitride, silicon nitride oxide, aluminumoxide, or tantalum oxide can be used. Alternatively, a stacked structureof these insulators can be used. Note that silicon oxynitride refers toa substance which contains more oxygen than nitrogen and containsoxygen, nitrogen, silicon, and hydrogen at given concentrations rangingfrom 55 atomic % to 65 atomic %, 1 atomic % to 20 atomic %, atomic % to35 atomic %, and 0.1 atomic % to 10 atomic %, respectively, where thetotal percentage of atoms is 100 atomic %. Further, silicon nitrideoxide refers to a substance which contains more nitrogen than oxygen andcontains oxygen, nitrogen, silicon, and hydrogen at given concentrationsranging from 15 atomic % to atomic %, 20 atomic % to 35 atomic %, 25atomic % to 35 atomic %, and 15 atomic % to 25 atomic %, respectively,where the total percentage of atoms is 100 atomic %.

As the oxide semiconductor layer 103, an oxide semiconductor such as anIn—Ga—Zn—O-based oxide semiconductor, an In—Sn—Zn—O-based oxidesemiconductor, an In—Zn—O-based oxide semiconductor, a Sn—Zn—O-basedoxide semiconductor, an In—Sn—O-based oxide semiconductor, aGa—Zn—O-based oxide semiconductor, or a Zn—O-based oxide semiconductorcan be used. Alternatively, an oxide semiconductor formed by addingnitrogen (N) or silicon (Si) to any of the above oxide semiconductorscan be used. A stacked structure of these materials can also be used.

As the pair of conductive layers 104 a and 104 b, titanium (Ti), copper(Cu), zinc (Zn), aluminum (Al), or the like can be used. Alternatively,an alloy containing any of these metal elements can be used. A stackedstructure of these materials can also be used.

As the metal oxide layer 105, the same material as the pair ofconductive layers 104 a and 104 b can be used. Note that the metal oxidelayer 105 has higher oxygen concentration than the pair of conductivelayers 104 a and 104 b. That is, the metal oxide layer 105 contains thesame metal element as the pair of conductive layers 104 a and 104 b, andhas higher oxygen concentration than the pair of conductive layers 104 aand 104 b.

As the source electrode layer 107 a and the drain electrode layer 107 b,an element selected from aluminum (Al), copper (Cu), titanium (Ti),tantalum (Ta), tungsten (W), molybdenum (Mo), chromium (Cr), neodymium(Nd), and scandium (Sc), an alloy containing any of these elements, or anitride containing any of these elements can be used. A stackedstructure of these materials can also be used.

As the pair of oxide semiconductor layers 108 a and 108 b whose oxygenconcentration is lowered, the same material as the oxide semiconductorlayer 103 can be used. Note that the pair of oxide semiconductor layers108 a and 108 b whose oxygen concentration is lowered has lower oxygenconcentration than the oxide semiconductor layer 103. That is, the pairof oxide semiconductor layers 108 a and 108 b whose oxygen concentrationis lowered contains the same metal element as the oxide semiconductorlayer 103, and has lower oxygen concentration than the oxidesemiconductor layer 103.

As the pair of conductive layers 109 a and 109 b containing oxygen athigh concentration, the same material as the pair of conductive layers104 a and 104 b and the metal oxide layer 105 can be used. Note that thepair of conductive layers 109 a and 109 b containing oxygen at highconcentration has higher oxygen concentration than the pair ofconductive layers 104 a and 104 b, and has lower oxygen concentrationthan the metal oxide layer 105. That is, the pair of conductive layers109 a and 109 b containing oxygen at high concentration contains thesame metal element as the pair of conductive layers 104 a and 104 b andthe metal oxide layer 105, and has higher oxygen concentration than thepair of conductive layers 104 a and 104 b and lower oxygen concentrationthan the metal oxide layer 105.

The thin film transistor 150 in FIG. 1A includes the buffer layer 106between the oxide semiconductor layer 103 and each of the sourceelectrode layer 107 a and the drain electrode layer 107 b. The bufferlayer 106 includes the metal oxide layer 105, which is the insulator orthe semiconductor, over the middle portion of the oxide semiconductorlayer 103. The metal oxide layer 105 functions as a protective layer forsuppressing incorporation of impurities (such as hydrogen and moisture)into the oxide semiconductor layer 103. Therefore, in the thin filmtransistor 150, an increase in off current or negative shift of thethreshold voltage can be prevented.

The buffer layer 110 of the thin film transistor 151 in FIG. 1B includesthe metal oxide layer 105 for preventing an increase in off current ornegative shift of the threshold voltage, and the pair of oxidesemiconductor layers 108 a and 108 b whose oxygen concentration islowered and which are provided over the opposite end portions of theoxide semiconductor layer 103. The pair of oxide semiconductor layers108 a and 108 b whose oxygen concentration is lowered has lowerresistance than the oxide semiconductor layer 103. Therefore, ohmiccontact between the oxide semiconductor layer 103 and each of the sourceelectrode layer 107 a and the drain electrode layer 107 b can beobtained.

Embodiment 2

In this embodiment, one example of a method for manufacturing the thinfilm transistor described in Embodiment 1 will be described withreference to FIGS. 2A to 2D and FIGS. 3A to 3D.

Note that in this embodiment, the term “film” means something which isformed over the entire surface of a substrate and is to be processedinto a desired shape in a subsequent photolithography step or the like,and something before the processing. The term “layer” means somethingobtained by processing and shaping a “film” into a desired shape by aphotolithography step or the like, or something to be formed over theentire surface of a substrate.

A first conductive film 201 is formed over a substrate 200. The firstconductive film 201 can be formed by a thin film deposition methodtypified by a sputtering method, a vacuum evaporation method, a pulsedlaser deposition method, an ion plating method, a metal organic chemicalvapor deposition method, or the like. Next, a first resist 202 is formedover the first conductive film 201. FIG. 2A is a cross-sectional viewafter the above steps are completed.

Next, with use of the first resist 202 as a mask, the first conductivefilm 201 is selectively etched to form a gate electrode layer 203. Notethat the materials described in Embodiment 1 can be used for thesubstrate 200 and the first conductive film 201 (the gate electrodelayer 203); therefore, here, the above description is to be referred to.The first resist 202 is removed after the gate electrode layer 203 isformed. FIG. 2B is a cross-sectional view after the above steps arecompleted.

Next, a gate insulating layer 204 is formed over the substrate 200 andthe gate electrode layer 203. The gate insulating layer 204 can beformed by a thin film deposition method typified by a sputtering method,a vacuum evaporation method, a pulsed laser deposition method, an ionplating method, a metal organic chemical vapor deposition method, aplasma CVD method, or the like.

Next, an oxide semiconductor film 205 is formed. The oxide semiconductorfilm 205 can be formed by a thin film deposition method typified by asputtering method, a vacuum evaporation method, a pulsed laserdeposition method, an ion plating method, a metal organic chemical vapordeposition method, or the like. In the case where an In—Ga—Zn—O-basedoxide semiconductor film is formed by a sputtering method, it ispreferable to use a target made by sintering In₂O₃, Ga₂O₃, and ZnO. As asputtering gas, a rare gas typified by argon is used. One example of theformation conditions by sputtering is as follows: a target made bymixing and sintering In₂O₃, Ga₂O₃, and ZnO (1:1:1) is used; the pressureis 0.4 Pa; the direct current (DC) power source is 500 W; the flow rateof an argon gas is sccm; and the flow rate of an oxygen gas is 15 sccm.After the oxide semiconductor film 205 is formed, thermal treatment ispreferably performed at 100° C. to 600° C., typically 200° C. to 400° C.Through this thermal treatment, rearrangement at the atomic level occursin the oxide semiconductor film. The thermal treatment (includingoptical annealing) is important because strain energy which inhibitscarrier movement in the oxide semiconductor film 205 is released by thethermal treatment.

Next, a second conductive film 206 is formed over the oxidesemiconductor film 205. The second conductive film 206 can be formed bya thin film deposition method typified by a sputtering method, a vacuumevaporation method, a pulsed laser deposition method, an ion platingmethod, a metal organic chemical vapor deposition method, or the like.As a material for the second conductive film 206, titanium (Ti), copper(Cu), zinc (Zn), aluminum (Al), or the like can be used. Alternatively,an alloy containing any of these metal elements can be used. A stackedstructure of these materials can also be used. Next, a second resist 207is formed over the second conductive film 206. FIG. 2C is across-sectional view after the above steps are completed.

Next, with use of the second resist 207 as a mask, the oxidesemiconductor film 205 and the second conductive film 206 areselectively etched to form an oxide semiconductor layer 208 and aconductive layer 209. Note that the materials described in Embodiment 1can be used for the gate insulating layer 204 and the oxidesemiconductor film 205 (the oxide semiconductor layer 208); therefore,here, the above description is to be referred to. The second resist 207is removed after the oxide semiconductor layer 208 and the conductivelayer 209 are formed. FIG. 2D is a cross-sectional view after the abovesteps are completed.

Next, a third conductive film 210 is formed over the gate insulatinglayer 204 and the conductive layer 209. The third conductive film 210can be formed by a thin film deposition method typified by a sputteringmethod, a vacuum evaporation method, a pulsed laser deposition method,an ion plating method, a metal organic chemical vapor deposition method,or the like. Next, third resists 211 a and 211 b are formed over thethird conductive film 210. FIG. 3A is a cross-sectional view after theabove steps are completed.

Next, with use of the third resists 211 a and 211 b as masks, the thirdconductive film 210 is selectively etched to form a source electrodelayer 212 a and a drain electrode layer 212 b. Note that through thisetching step, a region (an exposed portion) of the conductive layer 209,which does not overlap with the source electrode layer 212 a or thedrain electrode layer 212 b, is partly etched, whereby a conductivelayer 213 having a recessed portion in the region (the exposed portion),which does not overlap with the source electrode layer 212 a or thedrain electrode layer 212 b, is formed. Note that the materialsdescribed in Embodiment 1 can be used for the third conductive film 210(the source electrode layer 212 a and the drain electrode layer 212 b);therefore, here, the above description is to be referred to. FIG. 3B isa cross-sectional view after the above steps are completed.

Next, with use of the third resists 211 a and 211 b as masks, oxidationtreatment is performed. As the oxidation treatment, thermal oxidationtreatment in an oxidizing atmosphere, plasma oxidation treatment, oxygenion implantation, or the like can be used. It is possible to performplural kinds of such treatment in combination; for example, thermaloxidation treatment in an oxidizing atmosphere is performed, and then,plasma oxidation treatment is performed. Note that as the oxidizingatmosphere in which the thermal oxidation treatment is performed, adried oxygen atmosphere, a mixed atmosphere of oxygen and a rare gas, anatmospheric atmosphere, or the like can be used. Through the oxidationtreatment, a middle portion (an exposed portion) of the conductive layer213 provided over the oxide semiconductor layer 208 is oxidized to forma metal oxide layer 214 which is an insulator or a semiconductor.Further, at the same time as the formation of the metal oxide layer 214,a pair of conductive layers 215 a and 215 b is formed over opposite endportions of the oxide semiconductor layer 208. Specifically, by thesource electrode layer 212 a, the drain electrode layer 212 b, and thethird resists 211 a and 211 b, a region (a non-exposed portion) of theconductive layer 213, which overlaps with the source electrode layer 212a or the drain electrode layer 212 b, is prevented from being oxidized.As a result, the pair of conductive layers 215 a and 215 b remains. Notethat the volume of the region oxidized through the oxidation treatmentis increased. That is, the volume of the metal oxide layer 214 is largerthan that of the middle portion of the conductive layer 213 before beingoxidized. FIG. 3C is a cross-sectional view after the above steps arecompleted. Through the steps described above, the thin film transistor150 illustrated in FIG. 1A is completed.

Note that the thin film transistor of this embodiment is not limited tohave the structure of FIG. 1A or FIG. 3C. Specifically, the thin filmtransistor having such a structure that only the region (the middleportion) of the conductive layer 213, which does not overlap with thesource electrode layer 212 a or the drain electrode layer 212 b, isoxidized through the oxidation treatment to form the metal oxide layer214 is illustrated in FIG. 1A and FIG. 3C; however, a thin filmtransistor in which another region is also oxidized is also included inthe category of the thin film transistor of this embodiment. Forexample, a thin film transistor having such a structure that sideportions of the source electrode layer 212 a and the drain electrodelayer 212 b, which are not covered with the third resists 211 a and 211b, respectively, are oxidized through the oxidation treatment is alsoincluded in the category of the thin film transistor of this embodiment.Note that in the case where the side portions of the source electrodelayer 212 a and the drain electrode layer 212 b are oxidized, theoxidation is performed on only surfaces of the side portions of thesource electrode layer 212 a and the drain electrode layer 212 b,whereby the source electrode layer 212 a and the drain electrode layer212 b can function as electrodes. Similarly, a thin film transistorhaving such a structure that the region (the non-exposed portion) of theconductive layer 213, which overlaps with the source electrode layer 212a or the drain electrode layer 212 b, is partly internally oxidized isalso included in the category of the thin film transistor of thisembodiment.

The thin film transistor in which the thickness of the metal oxide layer214 formed through the oxidation treatment is larger than that of thepair of conductive layers 215 a and 215 b is illustrated in FIG. 1A andFIG. 3C; however, a thin film transistor in which the thickness of themetal oxide layer 214 is smaller than that of the pair of conductivelayers 215 a and 215 b is also included in the category of the thin filmtransistor of this embodiment. Note that the metal oxide layer 214 isformed by performing oxidation treatment on the conductive layer 213having the recessed portion. The recessed portion is formed in theetching step for forming the source electrode layer 212 a and the drainelectrode layer 212 b. That is, the condition of the etching step forforming the source electrode layer 212 a and the drain electrode layer212 b is adjusted, whereby the thickness of the metal oxide layer 214can be adjusted. Specifically, the time of overetching for forming thesource electrode layer 212 a and the drain electrode layer 212 b isprolonged, whereby the recessed portion can be deepened. Accordingly,the thickness of the metal oxide layer 214 can be smaller than that ofthe pair of conductive layers 215 a and 215 b.

In the case where the thin film transistor 151 in FIG. 1B ismanufactured, thermal treatment at 100° C. to 600° C., typically 200° C.to 400° C. is performed in a subsequent step. Through the thermaltreatment, oxygen in the oxide semiconductor layer 208 is diffused intothe pair of conductive layers 215 a and 215 b. Note that when diffusionof oxygen into the pair of conductive layers 215 a and 215 b is comparedwith diffusion of oxygen into the metal oxide layer 214, the amount ofoxygen diffused into the pair of conductive layers 215 a and 215 b islarger than that into the metal oxide layer 214. Therefore, a pair ofoxide semiconductor layers 216 a and 216 b whose oxygen concentration islowered is formed over the opposite end portions of the oxidesemiconductor layer 208, and a pair of conductive layers 217 a and 217 bcontaining oxygen at high concentration is formed over the pair of oxidesemiconductor layers 216 a and 216 b whose oxygen concentration islowered. After that, the third resists 211 a and 211 b are removed. FIG.3D is a cross-sectional view after the above steps are completed.

Here, the manufacturing step in which thermal treatment performed in thecase where the thin film transistor 151 in FIG. 1B is manufactured isperformed after oxidation treatment is described; however, the timing ofthe thermal treatment is not particularly limited as long as it is afterformation of the second conductive film 206. This thermal treatment alsoallows rearrangement at the atomic level in the oxide semiconductorlayer 208.

In terms of characteristics of a thin film transistor to be formed,thermal treatment is preferably performed after oxidation treatment.This is because when thermal treatment is performed before oxidationtreatment (before formation of the metal oxide layer 214), oxidesemiconductor layers whose oxygen concentration is lowered are formednot only in upper portions of the opposite end portions of the oxidesemiconductor layer 208 but also over the entire area of the oxidesemiconductor layer 208; thus, off current of a thin film transistor tobe formed is increased.

In addition, in terms of a manufacturing process, as the oxidationtreatment, thermal oxidation treatment is preferably performed in anoxidizing atmosphere at a temperature at which the pair of oxidesemiconductor layers 216 a and 216 b whose oxygen concentration islowered and the pair of conductive layers 217 a and 217 b containingoxygen at high concentration are formed. This is because the metal oxidelayer 214, the pair of oxide semiconductor layers 216 a and 216 b whoseoxygen concentration is lowered, and the pair of conductive layers 217 aand 217 b containing oxygen at high concentration can be formed in thesame step. One example of conditions for treatment serving as theoxidation treatment and the thermal treatment is thermal oxidationtreatment at 350° C. for one hour in a dried oxygen atmosphere.

Further, in terms of reliability of a thin film transistor to be formed,thermal oxidation treatment and oxidation treatment are preferablyperformed in combination. When the thickness of the metal oxide layer214 is made larger, a function as a protective layer for suppressingincorporation of impurities (such as hydrogen and moisture) into theoxide semiconductor layer 208 can be improved.

In the thin film transistor 150, the metal oxide layer 214 having afunction of preventing an increase in off current or negative shift ofthe threshold voltage is formed on the basis of the conductive layer 209(the conductive layer 213) which is formed in the same step as the oxidesemiconductor layer 208; therefore, a high-performance thin filmtransistor can be efficiently formed. In a similar manner, in the thinfilm transistor 151, the metal oxide layer 214 having a function ofpreventing an increase in off current or negative shift of the thresholdvoltage is formed on the basis of the conductive layer 209 (theconductive layer 213) which is formed in the same step as the oxidesemiconductor layer 208, and the pair of oxide semiconductor layers 216a and 216 b whose oxygen concentration is lowered and which have afunction of realizing ohmic contact between the oxide semiconductorlayer 208 and each of the source electrode layer 212 a and the drainelectrode layer 212 b is formed by diffusing oxygen into the pair ofconductive layers 215 a and 215 b; therefore, a high-performance thinfilm transistor can be efficiently formed.

Embodiment 3

In this embodiment, one example of a semiconductor device in which thethin film transistor described in Embodiment 1 is used will bedescribed. Specifically, a liquid crystal display device in which thethin film transistor is used as a thin film transistor provided in apixel portion of an active matrix substrate will be described withreference to FIG. 4, FIG. 5, and FIG. 6. Next, the liquid crystaldisplay device will be described.

Note that in a semiconductor device, since a source and a drain of athin film transistor are switched with each other depending on theoperating condition or the like, it is difficult to determine which isthe source or the drain. Therefore, in this embodiment and the followingembodiments, one of a source electrode layer and a drain electrode layeris referred to as a first electrode layer and the other thereof isreferred to as a second electrode layer for distinction.

FIG. 4 is a top view illustrating one pixel of an active matrixsubstrate. Three sub-pixels are included in a pixel of a liquid crystaldisplay device of this embodiment. Each sub-pixel is provided with athin film transistor 300 and a pixel electrode 301 for applying voltageto a liquid crystal layer. The thin film transistor described inEmbodiment 1 can be applied to the thin film transistor 300 in FIG. 4.In a pixel portion, a plurality of pixels described above are provided.In addition, a plurality of gate wirings 302, a plurality of sourcewirings 303, and a plurality of capacitor wirings 304 are provided.

FIG. 5 is a cross-sectional view taken along line A-B of FIG. 4. A thinfilm transistor 450 in FIG. 5 is the thin film transistor in FIG. 1A.That is, the thin film transistor 450 includes a gate electrode layer401 provided over a substrate 400, a gate insulating layer 402 providedover the gate electrode layer 401, an oxide semiconductor layer 403provided over the gate insulating layer 402, a buffer layer 406including a pair of conductive layers 404 a and 404 b and a metal oxidelayer 405 and provided over the oxide semiconductor layer 403, a firstelectrode layer 407 a provided over the conductive layer 404 a, and asecond electrode layer 407 b provided over the conductive layer 404 b.

The materials described in Embodiment 1 and the manufacturing methoddescribed in Embodiment 2 can be applied to the substrate 400, the gateelectrode layer 401, the gate insulating layer 402, the oxidesemiconductor layer 403, the conductive layer 404 a, the conductivelayer 404 b, the metal oxide layer 405, the buffer layer 406, the firstelectrode layer 407 a, and the second electrode layer 407 b; therefore,here, the above description is to be referred to.

The sub-pixel includes a capacitor 451. The capacitor 451 includes acapacitor wiring 408 formed from the same material as the gate electrodelayer 401 of the thin film transistor 450, the gate insulating layer402, and the second electrode layer 407 b of the thin film transistor450, which extends to the sub-pixel.

An interlayer insulating layer 409 is provided over the thin filmtransistor 450 and the capacitor 451. The thin film transistor 450 inFIG. 5 is provided with the metal oxide layer 405 for suppressingincorporation of impurities (such as hydrogen and moisture) into theoxide semiconductor layer 403; therefore, various materials andmanufacturing methods can be used for the interlayer insulating layer409. For example, as the interlayer insulating layer 409, a siliconoxide layer, a silicon oxynitride layer, a silicon nitride layer, asilicon nitride oxide layer, or the like can be formed by a plasma CVDmethod or a sputtering method. Further, the interlayer insulating layer409 can also be formed by an application method such as a spin coatingmethod, using an organic material such as polyimide, polyamide,polyvinylphenol, benzocyclobutene, acrylic, or epoxy; a siloxanematerial such as a siloxane resin; an oxazole resin; or the like. Notethat a siloxane material corresponds to a material having a Si—O—Sibond. Siloxane has a skeleton structure with a bond of silicon (Si) andoxygen (O). As a substituent, an organic group (e.g., an alkyl group oraromatic hydrocarbon) or a fluoro group may be used. The organic groupmay include a fluoro group. The second electrode layer 407 b in the thinfilm transistor 450 is electrically connected to a pixel electrode 411through a contact hole 410 provided in the interlayer insulating layer409.

FIG. 6 is an equivalent circuit diagram corresponding to the sub-pixelin FIG. 4. A gate electrode of a thin film transistor 500 iselectrically connected to a gate wiring 501, and a first electrode ofthe thin film transistor 500 is electrically connected to a sourcewiring 502. One electrode of a capacitor 503 is electrically connectedto a second electrode of the thin film transistor 500, and the otherelectrode of the capacitor 503 is electrically connected to a capacitorwiring 504. A liquid crystal layer 505 to which voltage is appliedthrough a pixel electrode is electrically connected to the secondelectrode of the thin film transistor 500 and to one electrode of thecapacitor 503.

A liquid crystal display device includes a liquid crystal layer providedbetween an active matrix substrate and a counter substrate provided witha counter electrode on its surface. Alignment of liquid crystalmolecules included in the liquid crystal layer is controlled by voltageapplied between a pixel electrode of the active matrix substrate and thecounter electrode of the counter substrate. The liquid crystal moleculesof the liquid crystal layer are aligned to transmit or block lightemitted from a backlight, whereby the liquid crystal display device candisplay is images. In the liquid crystal display device, a thin filmtransistor in a pixel portion of the active matrix substrate is aswitching element which controls voltage applied to the liquid crystallayer.

In the liquid crystal display device of this embodiment, the thin filmtransistor 450 in which the metal oxide layer 405 is provided over theoxide semiconductor layer 403 is used as a thin film transistor in apixel portion of an active matrix substrate. The metal oxide layer 405functions as a protective layer for suppressing incorporation ofimpurities (such as hydrogen and moisture) into the oxide semiconductorlayer 403. Therefore, the material and manufacturing method of theinterlayer insulating layer 409 can be selected depending on thepurpose. As a result, a liquid crystal display device with highperformance or high reliability can be provided. Note that the liquidcrystal display device to which the thin film transistor in FIG. 1A isapplied is described here; however, the same effect can be obtained alsoin the case where the thin film transistor in FIG. 1B is applied.

Embodiment 4

In this embodiment, one example of a semiconductor device in which thethin film transistor described in Embodiment 1 is used will bedescribed. Specifically, a light-emitting display device in which thethin film transistor is applied to a thin film transistor provided in apixel portion of an active matrix substrate will be described withreference to FIG. 7, FIG. 8, and FIG. 9. Next, a light-emitting displaydevice of this embodiment will be described. Note that as a displayelement included in the light-emitting display device of thisembodiment, a light-emitting element utilizing electroluminescence isdescribed here. Light-emitting elements utilizing electroluminescenceare classified according to whether a light-emitting material is anorganic compound or an inorganic compound. In general, the former isreferred to as organic EL elements and the latter as inorganic ELelements.

In an organic EL element, by application of voltage to a light-emittingelement, electrons and holes are separately injected from a pair ofelectrodes into a layer containing a light-emitting organic compound,and current flows. Then, recombination of these carriers (the electronsand holes) causes the light-emitting organic compound to form an excitedstate and to emit light when it returns from the excited state to aground state. Due to such a mechanism, such a light-emitting element isreferred to as a current-excitation light-emitting element.

Inorganic EL elements are classified into a dispersion type inorganic ELelement and a thin-film type inorganic EL element, depending on theirelement structures. A dispersion type inorganic EL element has alight-emitting layer where particles of a light-emitting material aredispersed in a binder, and its light emission mechanism isdonor-acceptor recombination type light emission that utilizes a donorlevel and an acceptor level. A thin-film type inorganic EL element has astructure where a light-emitting layer is sandwiched between dielectriclayers, which are further sandwiched between electrodes, and its lightemission mechanism is localized type light emission that utilizesinner-shell electron transition of metal ions. Note that description ismade here using an organic EL element as a light-emitting element.

FIG. 7 is a top view illustrating one pixel of an active matrixsubstrate. Three sub-pixels are included in a pixel of a light-emittingdisplay device of this embodiment. Each sub-pixel is provided with thinfilm transistors 600 and 601 and a pixel electrode 602 for applyingvoltage to a light-emitting element (part of the pixel electrode 602 isnot illustrated for convenience). The thin film transistor described inEmbodiment 1 can be applied to the thin film transistors 600 and 601 inFIG. 6. A plurality of pixels described above are provided in a pixelportion. In addition, a plurality of gate wirings 603, a plurality ofsource wirings 604, and a plurality of power supply lines 605 areprovided. Note that the power supply line 605 is set to have a highpower supply potential VDD.

FIG. 8 is a cross-sectional view taken along lines C-D and E-F of FIG.7. Thin film transistors 750 and 751 each correspond to the thin filmtransistor in FIG. 1A. That is, the thin film transistors 750 and 751each include a gate electrode layer 701 provided over a substrate 700, agate insulating layer 702 provided over the gate electrode layer 701, anoxide semiconductor layer 703 provided over the gate insulating layer702, a buffer layer 706 including conductive layers 704 a and 704 b anda metal oxide layer 705 and provided over the oxide semiconductor layer703, a first electrode layer 707 a provided over the conductive layer704 a, and a second electrode layer 707 b provided over the conductivelayer 704 b.

The materials described in Embodiment 1 and the manufacturing methoddescribed in Embodiment 2 can be applied to the substrate 700, the gateelectrode layer 701, the gate insulating layer 702, the oxidesemiconductor layer 703, the conductive layer 704 a, the conductivelayer 704 b, the metal oxide layer 705, the buffer layer 706, the firstelectrode layer 707 a, and the second electrode layer 707 b; therefore,here, the above description is to be referred to.

The sub-pixel includes a capacitor 752. The capacitor 752 includes acapacitor wiring 708 formed from the same material as the gate electrodelayer 701 of the thin film transistors 750 and 751, the gate insulatinglayer 702, and the first electrode layer 707 a of the thin filmtransistor 751, which extends to the sub-pixel.

An interlayer insulating layer 709 is provided over the thin filmtransistors 750 and 751 and the capacitor 752. Each of the thin filmtransistors 750 and 751 in FIG. 8 is provided with the metal oxide layer705 for suppressing incorporation of impurities (such as hydrogen andmoisture) into the oxide semiconductor layer 703; therefore, variousmaterials and manufacturing methods can be used for the interlayerinsulating layer 709. For example, as the interlayer insulating layer709, a silicon oxide layer, a silicon oxynitride layer, a siliconnitride layer, a silicon nitride oxide layer, or the like can be formedby a plasma CVD method or a sputtering method. Further, the interlayerinsulating layer 709 can also be formed by an application method such asa spin coating method, using an organic material such as polyimide,polyamide, polyvinylphenol, benzocyclobutene, acrylic, or epoxy; asiloxane material such as a siloxane resin; an oxazole resin; or thelike. Note that a siloxane material corresponds to a material having aSi—O—Si bond. Siloxane has a skeleton structure with a bond of silicon(Si) and oxygen (O). As a substituent, an organic group (e.g., an alkylgroup or aromatic hydrocarbon) or a fluoro group may be used. Theorganic group may include a fluoro group. Contact holes 710 a, 710 b,and 710 c are provided in the interlayer insulating layer 709. Thesecond electrode layer 707 b of the thin film transistor 751 iselectrically connected to a pixel electrode 711 through the contact hole710 c.

FIG. 9 is an equivalent circuit diagram corresponding to the sub-pixelin FIG. 7. A gate electrode of a thin film transistor 800 iselectrically connected to a gate wiring 801, and a first electrode ofthe thin film transistor 800 is electrically connected to a sourcewiring 802. One electrode of a capacitor 803 is electrically connectedto a second electrode of the thin film transistor 800, and the otherelectrode of the capacitor 803 is electrically connected to a powersupply line 804. A gate electrode of a thin film transistor 805 iselectrically connected to the second electrode of the thin filmtransistor 800, and a first electrode of the thin film transistor 805 iselectrically connected to the power supply line 804 and the otherelectrode of the capacitor 803. An organic EL element 806 to whichvoltage is applied through a pixel electrode is electrically connectedto a second electrode of the thin film transistor 805.

A light-emitting display device includes an organic EL element providedover a pixel electrode of an active matrix substrate, and a commonelectrode provided over the organic EL element. Note that the commonelectrode is set to have a low power supply potential VSS. When voltagecorresponding to a potential difference between the high power supplypotential VDD supplied to the pixel electrode through the thin filmtransistor and the low power supply potential VSS supplied to the commonelectrode is applied to the organic EL element, current flows to theorganic EL element so that the organic EL element emits light. In thelight-emitting display device, a thin film transistor in a pixel portionof the active matrix substrate is a switching element which controlscurrent flowing in the organic EL element.

In the light-emitting display device of this embodiment, the thin filmtransistors 750 and 751 in each of which the metal oxide layer 705 isprovided over the oxide semiconductor layer 703 are used as thin filmtransistors in the pixel portion of the active matrix substrate. Themetal oxide layer 705 functions as a protective layer for suppressingincorporation of impurities (such as hydrogen and moisture) into theoxide semiconductor layer 703. Therefore, the material and manufacturingmethod of the interlayer insulating layer 709 can be selected dependingon the purpose. As a result, a light-emitting display device with highperformance or high reliability can be provided. Note that here, thelight-emitting display device to which the thin film transistor in FIG.1A is applied is described; however, the same effect can be obtainedalso in the case where the thin film transistor in FIG. 1B is applied.

Embodiment 5

In this embodiment, one example of a semiconductor device in which thethin film transistor described in Embodiment 1 is used will bedescribed. Specifically, an electronic paper in which the thin filmtransistor is applied to a thin film transistor provided for an activematrix substrate will be described with reference to FIG. 10. Next, anelectronic paper of this embodiment will be described.

FIG. 10 is a cross-sectional view of an active matrix electronic paper.A thin film transistor 950 provided over a first substrate (an activematrix substrate) 900 is the thin film transistor in FIG. 1A. That is,the thin film transistor 950 includes a gate electrode layer 901provided over the first substrate 900, a gate insulating layer 902provided over the gate electrode layer 901, an oxide semiconductor layer903 provided over the gate insulating layer 902, a buffer layer 906including a pair of conductive layers 904 a and 904 b and a metal oxidelayer 905 and provided over the oxide semiconductor layer 903, a firstelectrode layer 907 a provided over the conductive layer 904 a, and asecond electrode layer 907 b provided over the conductive layer 904 b.

The materials described in Embodiment 1 and the manufacturing methoddescribed in Embodiment 2 can be applied to the first substrate 900, thegate electrode layer 901, the gate insulating layer 902, the oxidesemiconductor layer 903, the conductive layer 904 a, the conductivelayer 904 b, the metal oxide layer 905, the buffer layer 906, the firstelectrode layer 907 a, and the second electrode layer 907 b; therefore,here, the above description is to be referred to.

An interlayer insulating layer 908 is provided over the thin filmtransistor 950. The thin film transistor 950 in FIG. 10 is provided withthe metal oxide layer 905 for suppressing incorporation of impurities(such as hydrogen and moisture) into the oxide semiconductor layer 903;therefore, various materials and manufacturing methods can be used forthe interlayer insulating layer 908. For example, as the interlayerinsulating layer 908, a silicon oxide layer, a silicon oxynitride layer,a silicon nitride layer, a silicon nitride oxide layer, or the like canbe formed by a plasma CVD method or a sputtering method. Further, theinterlayer insulating layer 908 can also be formed by an applicationmethod such as a spin coating method, using an organic material such aspolyimide, polyamide, polyvinylphenol, benzocyclobutene, acrylic, orepoxy; a siloxane material such as a siloxane resin; an oxazole resin;or the like. Note that a siloxane material corresponds to a materialhaving a Si—O—Si bond. Siloxane has a skeleton structure with a bond ofsilicon (Si) and oxygen (O). As a substituent, an organic group (e.g.,an alkyl group or aromatic hydrocarbon) or a fluoro group may be used.The organic group may include a fluoro group. A contact hole 909 isprovided in the interlayer insulating layer 908. The second electrodelayer 907 b of the thin film transistor 950 is electrically connected toa pixel electrode 910 through the contact hole 909.

Between the pixel electrode 910 and a common electrode 912 which isprovided for the second substrate 911, twisting balls 915 each having ablack region 913 a, a white region 913 b, and a cavity 914 around theregions which is filled with liquid are provided. A space around thetwisting balls 915 is filled with a filler 916 such as a resin.

An electronic paper in this embodiment employs a twisting ball displaymethod. Twisting balls each colored in black or white are providedbetween a pixel electrode and a common electrode in the electronicpaper. The twisting balls perform display in such a manner that theorientation is controlled by voltage application between the pixelelectrode provided for the first substrate and the common electrodeprovided for the second substrate. In the electronic paper, a thin filmtransistor provided for an active matrix substrate is a switchingelement controlling voltage which is applied to twisting balls.

In the light-emitting display device of this embodiment, the thin filmtransistor 950 in which the metal oxide layer 905 is provided over theoxide semiconductor layer 903 is used as a thin film transistor providedfor the active matrix substrate. The metal oxide layer 905 functions asa protective layer for suppressing incorporation of impurities (such ashydrogen and moisture) into the oxide semiconductor layer 903.Therefore, the material and manufacturing method of the interlayerinsulating layer 908 can be selected depending on the purpose. As aresult, an electronic paper with high performance or high reliabilitycan be provided. Note that the electronic paper to which the thin filmtransistor in FIG. 1A is applied is described here; however, the sameeffect can be obtained also in the case where the thin film transistorin FIG. 1B is applied.

Example 1

Here, calculation results of change in electron states of titanium andtitanium oxide depending on the difference in the content of oxygen,change in an electron state of an oxide semiconductor layer inaccordance with oxygen deficiency, behavior of oxygen in the vicinity ofa bonding interface between a titanium layer and an oxide semiconductorlayer under thermal treatment, and behavior of oxygen in the vicinity ofa bonding interface between a titanium oxide layer and an oxidesemiconductor layer under thermal treatment are described. Next, thethin film transistor which is described in Embodiment 1 and in whichtitanium is applied to a composition material of the buffer layer isexamined.

First, change in electron states of titanium and titanium oxidedepending on the difference in the content of oxygen is examined. Here,results of obtaining the energy state density of crystal structures oftitanium and a plurality of titanium oxides by structure optimization bythe first-principle calculation using a plane wave-pseudopotentialmethod based on density functional theory (DFT) are described.Specifically, graphs show the state density of Ti, TiO (NaCl type),Ti₂O₃ (Al₂O₃ type), TiO₂ (Anatase type), TiO₂ (Rutile type), and TiO₂(Brookite type) after structures thereof are optimized. Note that CASTEPwas used for a calculation program, and GGA-PBE was used for anexchange-correlation function.

FIGS. 11A, 11B, and 11C show the state density of Ti, TiO (NaCl type),and Ti₂O₃ (Al₂O₃ type), respectively. In FIGS. 11A to 11C, there is noband gap. That is, Ti, TiO (NaCl type), and Ti₂O₃ (Al₂O₃ type) areconductors.

FIGS. 12A, 12B, and 12C show the state density of TiO₂ (Anatase type),TiO₂ (Rutile type), and TiO₂ (Brookite type), respectively. In FIGS. 12Ato 12C, Fermi level (0 eV) is located in an upper end of the valenceband, and there is a band gap. That is, each of TiO₂ (Anatase type),TiO₂ (Rutile type), and TiO₂ (Brookite type) is an insulator or asemiconductor.

It is confirmed from FIGS. 11A to 11C and FIGS. 12A to 12C that titaniumkeeps a property of a conductor when a predetermined amount of oxygen orless is included therein, and titanium comes to have a property of aninsulator or a property of a semiconductor when a predetermined amountof oxygen or more is included therein.

Next, change in an electron state of an oxide semiconductor layer inaccordance with oxygen deficiency is examined. Here, calculation isperformed in the case where an In—Ga—Zn—O-based oxide semiconductormaterial (In:Ga:Zn:O=1:1:1:4) is used for an oxide semiconductor layer.

First, an amorphous structure of the In—Ga—Zn—O-based oxidesemiconductor was formed by a melt-quench method using classicalmolecular dynamics simulation. Note that the amorphous structure formedhere is as follows: the total number of atoms is 84; and the density is5.9 g/cm³. Born-Mayer-Huggins potential was used for the interatomicpotential between metal and oxygen and between oxygen and oxygen, andLennard-Jones potential was used for the interatomic potential betweenmetal and metal. NTV ensemble was used for calculation. MaterialsExplorer was used as a calculation program.

After that, annealing by first-principle molecular dynamics (hereinafteralso referred to as first-principle MD) using a planewave-pseudopotential method based on density functional theory (DFT) wasperformed at a room temperature (298 K) on the structure obtained by theabove calculation in order to optimize the structure. Then, the statedensity was calculated. In addition, first-principle MD calculation wasused for calculation, optimization of the structure was performed on astructure in which one of oxygen atoms was removed randomly (a structurewith oxygen deficiency), and the state density was calculated. Note thatCASTEP was used as a calculation program; and GGA-PBE, anexchange-correlation function. First-principle MD was used forcalculation by using NTV ensemble.

FIGS. 13A and 13B each show the state density of the In—Ga—Zn—O-basedoxide semiconductor obtained by the above calculation. FIG. 13A showsthe state density of a structure without oxygen deficiency, and FIG. 13Bshows the state density of a structure with oxygen deficiency. In FIG.13A, Fermi level (0 eV) is located in an upper end of the valence bandand there is a band gap, while in FIG. 13B, Fermi level (0 eV) islocated in the conduction band. That is, it is confirmed that thestructure with oxygen deficiency has lower resistance than the structurewithout oxygen deficiency.

Next, behavior of oxygen in the vicinity of a bonding interface betweena titanium layer and an oxide semiconductor layer under thermaltreatment is examined. Here, titanium was deposited over the amorphousstructure of the In—Ga—Zn—O-based oxide semiconductor obtained by theabove first-principle calculation, and optimization of the structure wasperformed. Then, first-principle MD was used for calculation by usingNTV ensemble. CASTEP was used as a calculation program; and GGA-PBE, anexchange-correlation function. The temperature was set at 350° C. (623K).

FIGS. 14A and 14B show the structures before and after first-principleMD. FIG. 14A shows the structure before first-principle MD, and FIG. 14Bshows the structure after first-principle MD. Further, FIG. 15 shows thedensity of titanium and oxygen in the c-axis direction before and afterfirst-principle MD. FIG. 15 shows the density distribution obtained byassigning each atom in FIGS. 14A and 14B Gaussian distribution densityand summing all the atoms. In FIG. 15, the horizontal axis representsthe atom density, and the vertical axis represents the c-axis. Curves inFIG. 15 represent the density of titanium before first-principle MD(Ti_before), the density of titanium after first-principle MD(Ti_after), the density of oxygen before first-principle MD (O_before),and the density of oxygen after first-principle (O_after). It is foundfrom FIG. 15 that O_after is shifted toward the positive direction ofthe c-axis as compared with O_before, and the concentration of oxygencontained in titanium after first-principle MD is increased as comparedwith that before first-principle MD. That is, it is found that throughthe thermal treatment at 350° C. (623 K), oxygen in the oxidesemiconductor layer is diffused into the titanium layer.

Next, behavior of oxygen in the vicinity of a bonding interface betweenthe titanium oxide (here, TiO₂ (Rutile type) was used) layer and theoxide semiconductor layer under thermal treatment is examined. Here,TiO₂ (Rutile type) was deposited over the amorphous structure of theIn—Ga—Zn—O-based oxide semiconductor obtained by the first-principlecalculation, and optimization of the structure was performed, and then,first-principle MD was used for calculation by using NTV ensemble.CASTEP was used as a calculation program; and GGA-PBE, anexchange-correlation function. The temperature was set at 700° C. (973K).

FIGS. 16A and 16B show the structures before and after first-principleMD. FIG. 16A shows the structure before first-principle MD, and FIG. 16Bshows the structure after first-principle MD. Further, FIG. 17 shows thedensity of titanium and oxygen in the c-axis direction before and afterfirst-principle MD. FIG. 17 shows the density distribution obtained byassigning each atom in FIGS. 16A and 16B Gaussian distribution densityand summing all the atoms. In FIG. 17, the horizontal axis representsthe atom density, and the vertical axis represents the c-axis. Curves inFIG. 17 represent the density of titanium before first-principle MD(Ti_before), the density of titanium after first-principle MD(Ti_after), the density of oxygen before first-principle MD (O_before),and the density of oxygen after first-principle MD (O_after). Unlike inFIG. 15, there is not a large difference between O_after and O_before inFIG. 17. That is, it is found that even when the thermal treatment at700° C. (973 K) is performed, diffusion of oxygen between the oxidesemiconductor layer and the TiO₂ (Rutile type) layer is not activelyperformed as compared with diffusion of oxygen between the oxidesemiconductor layer and the titanium layer at 350° C.

Calculation results performed in this example are summarized below.

It is found from FIGS. 11A to 11C and FIGS. 12A to 12C that a pluralityof titanium oxides have different electron states, and when the oxygenconcentration is increased, the titanium oxide comes to have a propertyof an insulator or a property of a semiconductor. Specifically, it isfound that TiO (NaCl type) and Ti₂O₃ (Al₂O₃ type) are conductors, andeach of TiO₂ (Anatase type), TiO₂ (Rutile type), and TiO₂ (Brookitetype) is an insulator or a semiconductor. That is, it is found that atitanium oxide comes to have a property of an insulator or a property ofa semiconductor when the content of oxygen is large, and its electrondensity is changed depending on the ratio of oxygen.

It is found from FIGS. 13A and 13B that, when the In—Ga—Zn—O-based oxidesemiconductor has the structure with oxygen deficiency, the electronstate is changed and the resistance is lowered. Note that in FIGS. 13Aand 13B, the amorphous structure whose total number of atoms is 84(In:Ga:Zn:O=1:1:1:4) is compared with the structure in which one ofoxygen atoms is removed from the amorphous structure. In other words,the structure whose oxygen concentration is about 57.1 atomic % (48 (thenumber of oxygen atoms)/84 (the number of total atoms)) is compared withthe structure whose oxygen concentration is about 56.6 atomic % (47 (thenumber of oxygen atoms)/83 (the number of total atoms)). Accordingly,the In—Ga—Zn—O-based oxide semiconductor is a material whose change inoxygen concentration greatly affects the electron state as compared withtitanium described above.

It is found from FIGS. 14A and 14B and FIG. 15 that oxygen in theIn—Ga—Zn—O-based oxide semiconductor layer is diffused into the titaniumlayer when the thermal treatment at 350° C. is performed on the titaniumlayer and the In—Ga—Zn—O-based oxide semiconductor layer. In otherwords, it is found that the titanium layer containing oxygen at higherconcentration than the titanium layer before the thermal treatment andthe oxide semiconductor layer whose oxygen concentration is lower thanthe oxide semiconductor before the thermal treatment are formed throughthe thermal treatment. It is thought that, when considering an effect ofchange in the oxygen concentration of the titanium layer and theIn—Ga—Zn—O-based oxide semiconductor layer on the electron statesthereof, the resistance of the titanium layer containing oxygen at highconcentration is not increased so much as compared with that of thetitanium layer, and the resistance of the oxide semiconductor layerwhose oxygen concentration is lowered is reduced as compared with thatof the oxide semiconductor layer.

It is found from FIGS. 16A and 16B and FIG. 17 that even when thethermal treatment at a temperature as high as 700° C. is performed on astack of the TiO₂ (Rutile type) layer and the In—Ga—Zn—O-based oxidesemiconductor layer, diffusion of oxygen from the oxide semiconductorlayer to the titanium layer is not actively performed as compared withdiffusion of oxygen from the oxide semiconductor layer to the titaniumlayer at 350° C. In other words, it is found that the oxidesemiconductor layer whose oxygen concentration is lower than that of thestack of the oxide semiconductor layer and the titanium layer is noteasily formed even when the thermal treatment is performed.

Next, the case where titanium is applied to the buffer layer in the thinfilm transistor described in Embodiment 1 is examined. By performingoxidation treatment on the titanium layer, a titanium oxide such as TiO₂(Anatase type), TiO₂ (Rutile type), or TiO₂ (Brookite type) which is aninsulator or a semiconductor is formed to be used as a metal oxide layerincluded in the buffer layer. By performing thermal treatment at 350°C., oxygen in the oxide semiconductor layer is diffused into thetitanium layer, whereby a titanium layer containing oxygen at highconcentration and an oxide semiconductor layer whose oxygenconcentration is lowered are formed. Therefore, the resistance of theoxide semiconductor layer can be effectively reduced, and ohmic contactbetween the oxide semiconductor layer and each of the source electrodelayer and the drain electrode layer can be obtained through the bufferlayer. In addition, diffusion of oxygen does not easily occur at aninterface between the oxide semiconductor layer and the metal oxidelayer, as compared with at an interface between an oxide semiconductorlayer and a conductive layer. Accordingly, the oxide semiconductor layerwhose oxygen concentration is lowered and whose resistance is reduced isnot easily formed at the interface between the oxide semiconductor layerand the metal oxide layer, whereby an increase in off current of thethin film transistor can be suppressed.

From the above, it is confirmed that titanium is preferably used as amaterial applied to the buffer layer in the thin film transistordescribed in Embodiment 1.

This application is based on Japanese Patent Application serial No.2009-037912 filed with Japan Patent Office on Feb. 20, 2009, the entirecontents of which are hereby incorporated by reference.

What is claimed is:
 1. A semiconductor device comprising: a first transistor; a second transistor; a capacitor; and a light-emitting element, wherein each of the first transistor and the second transistor comprises: a gate electrode comprising titanium and molybdenum; a first insulating layer; an oxide semiconductor layer overlapping with the gate electrode with the first insulating layer interposed therebetween; a source electrode electrically connected to the oxide semiconductor layer, the source electrode comprising titanium and molybdenum; and a drain electrode electrically connected to the oxide semiconductor layer, the drain electrode comprising titanium and molybdenum, wherein one electrode of the capacitor is electrically connected to one of the source electrode and the drain electrode of the first transistor and the gate electrode of the second transistor, wherein the light-emitting element is electrically connected to one of the source electrode and the drain electrode of the second transistor, and wherein the first insulating layer includes a contact hole in a region where the first transistor, the second transistor, and the capacitor are not overlapped with each other.
 2. A semiconductor device comprising: a first transistor; a second transistor; a capacitor; and a light-emitting element, wherein each of the first transistor and the second transistor comprises: a gate electrode comprising titanium and molybdenum; a first insulating layer; an oxide semiconductor layer overlapping with the gate electrode with the first insulating layer interposed therebetween; a source electrode electrically connected to the oxide semiconductor layer, the source electrode comprising titanium and molybdenum; and a drain electrode electrically connected to the oxide semiconductor layer, the drain electrode comprising titanium and molybdenum, wherein one electrode of the capacitor is electrically connected to one of the source electrode and the drain electrode of the first transistor and the gate electrode of the second transistor, wherein the light-emitting element is electrically connected to one of the source electrode and the drain electrode of the second transistor, wherein the oxide semiconductor layer comprises indium, gallium, and zinc, and wherein the first insulating layer includes a contact hole in a region where the first transistor, the second transistor, and the capacitor are not overlapped with each other.
 3. The semiconductor device according to claim 1, wherein the gate electrode of the second transistor is electrically connected to the one of the source electrode and the drain electrode of the first transistor through the contact hole.
 4. The semiconductor device according to claim 1, wherein the first insulating layer is between the one electrode of the capacitor and the other of the source electrode and the drain electrode of the second transistor.
 5. The semiconductor device according to claim 1, further comprising an interlayer insulating layer over the first transistor, the capacitor, and the second transistor.
 6. The semiconductor device according to claim 1, wherein the one electrode of the capacitor is overlapped with a pixel electrode.
 7. The semiconductor device according to claim 2, wherein the gate electrode of the second transistor is electrically connected to the one of the source electrode and the drain electrode of the first transistor through the contact hole.
 8. The semiconductor device according to claim 2, wherein the first insulating layer is between the one electrode of the capacitor and the other of the source electrode and the drain electrode of the second transistor.
 9. The semiconductor device according to claim 2, further comprising an interlayer insulating layer over the first transistor, the capacitor, and the second transistor.
 10. The semiconductor device according to claim 2, wherein the one electrode of the capacitor is overlapped with a pixel electrode. 